Burnt Wire

Sounds like my experience with my 1985 M-B diesel car. I happened to crank the engine while I had the dash cluster pulled out and saw smoke coming from the speedometer cable. That ain't right since the starter's return current should go thru the engine block to the frame and then the battery (BAT- cable goes to frame). That path is thru a thick aluminum cable from transmission bolt to frame at the bottom. That recalled stories of 1960's homes built with cheaper aluminum wiring catching fire from corroded connections and resulting heat. I removed that ground cable which looked pristine, sanded both ends, coated with Si grease, and bolted tight. No more smoking speedo cable. Similarly, when my 1969 slant alternator wouldn't charge, nor would a new one, I found the problem was the case to block bracket electrical interface (a ground wire from case is better). Aluminum oxide is that white powder and quickly forms almost invisibly on fresh aluminum. Also known as "beach sand" it is a non-conductor as are all ceramics, but does make a good sandpaper (sapphire as a pure crystal, 2nd hardest to diamond).